Geothermal comes ultimately from the gravitational energy of the earth itself compressing and heating it
One thing that’s at least 97-percent certain is that radioactive decay supplies only about half the Earth’s heat. Other sources – primordial heat left over from the planet’s formation, and possibly others as well – must account for the rest.
A surprising amount of geothermal energy comes from radioactive decay. Gravitational binding energy is indeed very large, but much of that heat has already radiated away before a solid crust formed.